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Triple electron entanglement boosts quantum computing

By Will Knight

A new semiconductor-based technique for entangling multiple electrons could mark a significant step towards the development the first fully-functional quantum computer.

Roberto Merlin and colleagues at the University of Michigan, along with Jacek Furdyna of the University of Notre Dame in Indiana, used ultra-fast laser pulses to entangle three electrons in a quantum well made of semiconducting cadmium telluride.

Most schemes for entangling multiple quantum bits, or qubits, have involved ions or photons and have used complicated laboratory set-ups. By contrast, the new scheme uses electrons, the building blocks of conventional computers. Also, although others have entangled two electrons, entangling more qubits is crucial to developing an effective quantum logic circuits.

“Most papers on quantum computing are theoretical, we proposed the method and show that it works,” Merlin told New Scientist.

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Bill Munro, of Hewlett Packard’s research laboratory in Bristol, UK, says&colon; “For a lot of quantum computation you want to be able to entangle many things together. I like the proposal a lot. In some ways it shows the possibilities for the future.”

Femtosecond pulses

Electrons normally have one of two spin states. But quantum physics can be exploited to give an electron more than one spin state at the same time. Electrons can also be “entangled” so that interacting with one also affects the other, no matter how far apart they are.

Exploiting such quantum weirdness, a quantum computer would be able to perform many computations at once, making it vastly more powerful than a conventional computer.

In the latest research, the scientists used laser pulses 50 to 100 femtoseconds (10-15s) apart to generate excitons within the cadmium telluride. An excitons is a pair of charge carriers – a negative electron, which has had its energy boosted by the laser pulses, and the positive “holes” this leaves behind.

Each exciton interacts with extra electrons present in the semiconductor because of its inherent impurities. These “donor bound” electrons are affected by the exciton’s magnetic field and, if more than one electron is close to this exciton, they become entangled.

The experiments showed that three electrons could be entangled in this way. This was verified by a threefold increase in the signal that would be expected if an exciton had interacted with just one electron.

Merlin adds that, if individual excitons could be addressed using laser pulses, the entanglement could be extended to many more electrons.